In recent years, optical elements made of glass are utilized in a wide range of applications as a lens for digital cameras, an optical pick up lens for DVDs, a camera lens for cell phones and a coupling lens for optical communication. As such glass optical elements, molded glass objects manufactured by press-molding of a glass material by use of a molding die are widely utilized.
As a manufacturing method of molded glass objects, known are a method in which a glass preform having a predetermined weight and form is prepared in advance and said glass preform is heated together with a molding die to be subjected to press-molding (hereinafter, also referred to as “a reheat press method”), and a method in which a dropped molten glass drop is received by an under mold and the received molten glass drop is subjected to press-molding to prepare a molded glass object (hereinafter, also referred to as “a liquid drop molding method”). Particularly, the latter method gathers attention for a very short cycle time for molding. By that method, it is possible to manufacture a molded glass object directly from a molten glass drop without repeated heating and cooling of the molding die.
However, in either of the above-described methods, there has been a problem that a molded glass object may have defects such as dents and wrinkles created by ambient gas caught between a glass material and an upper mold in a process where the glass material is deformed along the molding surface of the upper mold at the time of press-molding.
To overcome such a problem, there has been proposed a method to prevent ambient gas from being caught by providing a chamfering treatment on the outer edge of a glass preform to make the outer edge of a glass preform not contact with a molding die (for example, Japanese Laid-Open Patent Application Publication No. H02-102134), and a method to perform setting of such as a molding die and a glass preform under vacuum (for example, Japanese Laid-Open Patent Application Publication No. H06-340433).
However, in the liquid drop molding method, it is impossible to employ a method using a glass preform having a complicated shape as described in Japanese Laid-Open Patent Application Publication No. H02-102134 because a molten glass drop having been received by an under mold is subjected to press-molding in the liquid drop molding method. Further, in the case of applying a method described in Japanese Laid-Open Patent Application Publication No. H06-340433 to the liquid drop molding method, there was a problem of very large facilities being required to perform the whole processes in vacuum, from dropping of a molten glass drop to press-molding.